Heretofore, there have been manufactured many display devices utilizing a refractive index anisotropy, a dielectric anisotropy and the like which are characteristics of a liquid crystal. The liquid crystal display devices have widely been utilized in watches, electronic calculators, word processors, televisions and the like, and the demand of the liquid crystal display devices is increasing year by year. A liquid crystal phase lies between a solid phase and a liquid phase. This liquid crystal phase can be roughly classified into a nematic phase, a smectic phase and a cholesteric phase. The display devices utilizing the nematic phase among these liquid crystal phases have been most extensively utilized of late.
As display systems in which an electro-optical effect of the liquid crystal is applied to the liquid crystal display, there are a twisted nematic type (TN type), a dynamic scattering type (DS type), a guest-host type (GH type), a DAP type and the like. In particular, the coloring of the liquid crystal display has remarkably progressed in recent years. In the case of the TN type, display systems for the color display are mainly a thin film transistor type (TFT type) and a super-twisted nematic type (STN type), and the display devices having these display systems have been mass-produced.
Many liquid crystal compounds inclusive of compounds which are now under research steps are known, but there does not exist any liquid crystal compound which can be used in the display devices singly instead of a mixture of the liquid crystal compounds. A property which is required for the liquid crystal compound for use in the display device is that the compound shows the liquid crystal phase in the widest possible natural temperature range including typical room temperature at which the display device is most often used.
Furthermore, it is also be required that the liquid crystal compound is sufficiently stable to environmental factors and have physical properties enough to drive the display device. However, any liquid crystal compound which can meet all of these requirements singly has not been found so far.
At present, several kinds of liquid crystal compounds or non-liquid crystal compounds are mixed to prepare a composition having the above characteristics, and the thus prepared composition is put to practical use as the liquid crystal material for the display. The liquid crystal composition is required to be stable to water, light, heat, air and the like present under a usual use environment, as described above. In addition, it is also required that the liquid crystal composition is stable to electric field and electromagnetic radiation and the mixed respective liquid crystal compounds are chemically stable under the use environment.
Furthermore, physical values of a refractive index anisotropy (.DELTA.n), a dielectric anisotropy (.DELTA..epsilon.), a viscosity (.eta.), a conductivity, an elastic constant ratio K.sub.33 /K.sub.11 (K.sub.33 : a bend constant, K.sub.11 : a spray elastic constant) and the like of the liquid crystal composition are required to be suitable in compliance with the display system and a device shape. Moreover, it is important that the respective components in the liquid composition have a mutually good solubility.
Among these physical values, a wide liquid crystal phase temperature range, the large dielectric anisotropy, the high elastic constant ratio K.sub.33 /K.sub.11 and the like are mainly required for the liquid crystal compounds which can be used in the TFT type display system. In this display system, the improvement of contrast is necessary with the progress of the coloring of the display particularly in recent years. In order to improve the contrast, it is necessary and essential that the liquid crystal compounds have the large refractive index anisotropy value. Furthermore, it is also necessary to lower the dependence of a temperature on a voltage retention. The voltage retention of the liquid crystal compounds is liable to lower with the rise of the temperature. With regard to the displays which have now been used, a back light irradiation type is more prevalent than a reflective type, and in this back light irradiation type, an environment under which a display panel is arranged has a higher temperature than room temperature owing to the irradiation heat of a back light. Under such circumstances, the temperature rise of the liquid crystal compounds is not avoidable. With the temperature rise of the liquid crystal compounds, their voltage retention lowers, and for example, in a normally white mode, a contrast change of from black to white takes place. In consequence, a sharp screen display is impossible.
Many liquid crystal compounds having the low dependence of the temperature on the voltage retention have been heretofore developed. As the suitable compounds, there are well known to a person skilled in the art, for example, compounds having a 3,4,5-trifluorophenyl group which are represented by the following formula (a) and which are mentioned in Japanese Patent Application Laid-open No. 233626/1990. By using any of these compounds as the components of the liquid crystal composition, it is possible to obtain the high voltage retention. However, for the liquid crystal compounds which can be used for the high-speed screen display of a dynamic image or the like, there have been required the expansion of a liquid crystal phase temperature range, the deterioration of viscosity, and a high mutual solubility of the liquid crystal compounds at a low temperature in the case that they are used as the components of the liquid crystal composition. ##STR2##